Four-wire system with echo suppressors



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wwf/M ATTORN EYS Patented May 19, 1936 UNITED STATES FOUR-WIRE SYSTEM WITH ECHO SUPPRESSORS Hans Mayer, Berlin-Charlottenburg, Germany,

assignor to Siemens & Halske, Aktiengesellschaft, Siemensstadt, near Berlin, Germany, a corporation of Germany Application September 11, 1933, Serial No. 689,014 In Germany September 10, 1932` 12 Claims.

(Granted under the provisions of sec. 14, act of March 2, 1927; 357 0. G.

This invention relates to four-wire telephone transmission systems, and more particularly to methods of and means for suppressing echoes in such systems.

One of the objects of the invention is to prevent transmission defects such as intermittent lock-outs caused by the operation of the echosuppressors in said systems.

It is well known to al1 who are skilled in the art that it is advantageous in long-distance four- Wire telephone transmission circuits-consisting either of only one very long four-wire line or of two or more four-wire lines connected in tandemto arrange the echo-Suppressors in crosssituation, i. e. in such a manner that the branch point of each of the Suppressors follows the suppression point of the other suppressor, .and to locate these echo-Suppressors near the terminals of the four-wire system, for example at a distance of one or more repeater sections from the terminals. This is done in order to limit the hang-over time of the echo-Suppressors as far as possible and, therefore, to enable the hearing subscriber to speak to the calling one during short pauses of conversation. On the other hand, however, this arrangement has the disadvantage of increasing, with growing transmission time between the two end terminals of the four-wire system, the chance of intermittent mutual lockouts of the two subscribers and, therefore, to restrict the highest possible distance between the subscribers. If, in a long-distance four-wire system vwth one echo-suppressor near each end, the two subscribers begin to talk nearly at the same instant, the voice currents from one party will block, on the far end of the system, the return path, but, a short time later, they will be blocked themselves by that part of thespeech currents of the other party which have already passed through the return path. The speech currents in the outgoing path are, therefore, blocked only for a limited period, so that they will again block the return path, and so on. This counter action of the echo-Suppressors which is explained more fully hereinafter. results in an intermittent mutual lock-out, which continues as long as the two subscribers continue to talk simultaneously.

It has already been attempted to overcome this diiculty by placing additional apparatus near the echo-Suppressors, each of said apparatus causing the voice currents in the outgoing path of the four-wire system to prevent the disabling of this path by currents in the incoming path of simultaneous or later arrival. The arrangement of such apparatus, however, is very complicated and expensive in that it is necessary to use special delay devices in order to conform or attune the operating time of the additional apparatus to the transmission time of the four-wire system. Moreover, there exists the disadvantage 5 that, in the case of connecting some four-wire lines in tandem, improper operation is possible because `of the fact that the transmission time of the whole four-wire circuit is greater than the time to which the delay circuits of the apparatus at the terminals of the combined four-wire circuit are adjusted or attuned.

According to the invention, all these difliculties are removed by providing, besides the two cross-situated echo-Suppressors near the terminals of the four-wire system and between them,

a further pair of cross-situated echo-Suppressors, preferably near the electrical midpoint of the four-wire connection. This midpoint may be defined as the point from which the transmission 2o times to the two subscribers are substantially equal.

The invention will be more fully understood from the following description when read in connection with the accompanying drawing, wherein Fig. 1 is a simplified diagram showing an arrangement of echo-Suppressors heretofore used;

Fig. 2 is a diagram showing a preferred arrangement of echo-Suppressors according to this invention;

Figs. 3 and 4 are simplied diagrams showing how the known four-wire circuit of Fig. 1 may be embodied, in a well known manner, in the tandem-connection of some four-wire lines;

Figs. 5 and 6 are diagrams showing how the 35 arrangement according to Fig. 2 may be applied to circuits of the nature shown in Figs. 3 and 4; and

Fig. 8 is a diagram showing further modications of the invention adapted for use in a general toll switching plan which is illustrated schematically by Fig. 7.

Referring to Fig. 1, there is shown diagrammatically a long four-wire circuit with two echosuppressors ES1 and ESz which are cross-situated 45 and located near the terminals of the four-wire circuit. At these terminals, the subscriber stations T1 and T2, respectively, may be connected by means of two-wire circuits well known in the art. The branch point of the echo-suppressor ES1, i. e. the point at which the suppressor input is branched from the lower one-way path, is designated as A1 whereas P1 is the suppression point, i. e. the point at which the currents in the upper path are suppressed. In the same way,

A2 and P2 are the branch point and the suppression point, respectively, of the echo-suppressor ES2. The operating time of the echo-Suppressors may be shorter than the shortest transmission time of the echo currents from the branch point over the terminal to the suppression point in the other path, whereas the hang-over time is to be greater than the longest echo time.

Now, it may be assumed that the transmission time from the suppressing point of the one echosuppressor to the branch point of the other, that is the transmission time from P1 to A2 and from P2 to A1, respectively, will be very great, for example of the order of 1 second, and that the subscriber at T1 Will begin to speak at the time t=0, and the other subscriber at T2 at t=0.5 second. The voice currents of the subscriber at T1 will thus reach the point A2 and block the return path at the point P2 approximately at the time t=l second. In the meantime, a part of the voice currents of the subscriber at T2 have already passed the point P2 so that this part. of voice currents after reaching the point A1, will actuate the echo-suppressor ES1 and therefore suppress the voice currents of the party T1 at the point P1 for a time interval from about t=1.5 to t=2 seconds. This enforced interruption of the voice currents in the upper transmission path will have the effect that, after reaching the echosuppressor ES2 at A2, the lower path will be again opened for a short time interval approximately from t=2.5 to t=3 seconds so that a further part of the voice currents from T2 may pass through the lower path and actuate the echosuppressor ES1 again. This counter-action will be continued as long as both parties continue to speak, and will begin anew when the two subscribers, after observing the interruptions, will pause and then begin to speak again approximately at the same instant.

From this description it is obvious that the trouble will appear in each case when the time difference between the beginnings of the voice currents of the two parties is shorter than the transmission time over the main part of the four- Wire circuit. Thus, the frequency of the transmission defects will increase with an increasing of the transmission time. For this reason, it was heretofore necessary to restrict the transmission time of the main part of a four-wire connection to 0.1 second, and the total transmission time of the whole four-wire circuit tol about 0.2 second, assuming that the echo time at each terminal from the branch point to the suppression point of the associated echo-suppressor will be about 0.1=2 0.05 second.

Fig. 2 shows, in a simplified diagram, how to avoid the aforesaid trouble according to my invention. The arrangement is, in general, similar to that in Fig. 1 except that there is arranged, near the midpoint of the four-wire circuit, a further pair of cross-situated echo-Suppressors ESS and ES4, the distance between which may be very small. l

This arrangement has the advantage that, practically in each case, the subscriber who begins to speak will also get control of the entire connection. If, for instance, the party at T1 should start speaking a very short time earlier than the party at T2, then the voice currents of T1 Will reach the branch point A4 of the echo-suppressor ES4 earlier than the voice currents of T2 reach the suppression point P4 so that these later voice currents will nd their further path a1- ready blocked and, therefore, can not reach the echo-Suppressors ES3 and ES1. After a time period which is equal to the transmission time from the point A4 to A2, the Voice currents of T1 will reach the point A2 and also actuate the echosuppressor E82. Now, the lower path is blocked totally, and it Will be held blocked until the echosuppressor ESz, after the voice currents in the upper path have ceased, has returned to its normal state.

From this it follows that the beginning o-f the echo-suppression depends on the operation time of the midpoint echo-Suppressors E53 and ES4, and the end of the suppressor-action on the hang-over time of the terminal echo-Suppressors ES1 and ES2. The properties or characteristics of the echo-Suppressors may be chosen accordingly.

The description following hereinafter explains how to use my invention in cases where two or more four-wire lines are connected to form a combined long four-wire circuit. For the sake of simpler illustration, in Figs. 3 to 6 and 8 each one- Way path of a four-wire circuit which really comprises several one-way repeaters, is represented by a line with only one repeater symbol, and each echo-suppressor only by an arrow.

Fig. 3 shows two normal four-wire lines each of which contains two cross-situated echo-suppressors, one at the one terminal and the other at the other terminal of the associated four-Wire line. When connecting these two four-Wire lines in tandem at their neighbouring terminals the combined four-wire circuit Will comprise four echo-Suppressors I, 2, 3 and 4. The echo-suppressors I and l! at the end terminals are situated correctly whereas the echo-Suppressors 2 and 3 are situated in the Wrong direction so that a continuous mutual lock-out of the speaking subscribers is possible. When the two subscribers begin to speak nearly at the same instant, the voice currents in the upper path coming from the left will actuate the echo-suppressor 2 to disable the lower path, and the voice currents in this path coming from the right will block the upper path by actuating the echo-suppressor 3. Thus the subscribers might speak simultaneously for a long time without hearing each other.

For this reason, there had been resorted to heretofore the expedient of disconnecting or disabling totally the echo-Suppressors at the switching points, as shown diagrammatically by Fig. 4, so that only the two echo-Suppressors at the outer terminals of the combined circuit remained effective. This arrangement is essentially the same as the arrangement shown by Fig. 1, and the transmission time in the main part of the complete four-wire circuit is also great because of the summation of the transmission times of the four-wire lines which are connected. It will be, therefore, evident that, if the two subscribers at the ends of the connection start to speak simultaneously, there will appear the same trouble in the form of an intermittent mutual lock-out as explained above in relation to Fig. 1. i

Fig. 5 shows an example of how to avoid that trouble in accordance with the principle of the invention as shown by the fundamental design of Fig. 2. The two echo-Suppressors associated with the terminals at the switch point are not disconnected or disabled, in the manner heretofore available in practice, but reversed in action or effect by changing over their branch points and suppression points respectively. The reversed echo-suppressors 2 and 3 will then constitute an additional pair of cross-situated echosuppressors, preferably near the midpoint of the connection, corresponding to the showing in Fig. 2.

The reversal of the echo-Suppressors may be accomplished in various ways. It is possible to arrange, in the input circuit of the suppressor, a relay-operated change-over switch by which the said input circuit is connected in the normal position with one, and in the operating position with the other, of the two opposite transmission paths. In the output circuit of the suppressor, there may be provided another change-over switch by which is switched on, for example, in the normal position, the grid circuit of a repeater in one of the two opposite paths, and in the operating position, the grid circuit of a repeater in the other path. Simultaneously, the grid circuit which is not switched on to the suppressor output may be held on a xed base potential in order to secure good amplication. In

vthe case of echo-Suppressors with relay operation, each of the two paths near a terminal may be associated with a relay whose contacts are switched into the said path whereas further switching means are provided to switch on either the one or the other of the two relays to the output circuit of the echo-suppressor.

Fig. 6 shows a further embodiment of the invention. Instead of reversing the echosuppressors near the connecting points, there are provided two oppositely directed echo-Suppressors at each terminal one of which is operable in the normal state whereas, in the case of connecting two terminals, at each of these terminals the echo-suppressor which is operable in the normal state will be disabled and the other one, in the opposite direction, will be made eiective. full lines represent the echo-Suppressors operable in the normal state whereas the oppositely directed echo-Suppressors are represented by dotted lines. When connecting the two fourwire lines in the manner shown, the reversing of the echo-Suppressors has to be eiected in such a way that the Suppressors 2 and 3 will be disabled or disconnected and the Suppressors 2 and 3 rendered effective orswitched on'. It is evident that in this case the completed circuit arrangement is effectively the same as shown in Figs. 2 and 5.

The reversing or changing-over of the echosuppressors is to be controlled from the switch point at which the associated terminals are switched together. The control may be eiected by a direct or alternating current which is to be transmitted over one of the speech paths or over a further pilot circuit, and the transmission of the said control current may be effected either automatically or by manual switching.

The invention is also applicable when more than two four-wire lines are connected, or switched together as is the case when building up through-connections in accordance with the general toll switching plan. According to this plan, a plurality of so-called toll centers each of which may comprise the ofces of a rural district, are connected in common to a switching ofllce of higher order, the so-called primary outlet. A plurality of such primary outlets may be combined together by a toll oice of the next higher order, a so-called regional center, whereas a plurality of such regional centers are connected to the highest-grade toll oice which may be called a world center.

The

ent centers of the same order may be called cross-connections, such as EF--EF, 4VF-VF, DF--DF, and VVF-WF. The connections from a center to the associated center of higher or lower grade may be called' series connections, such as EF-VF, VF-DR and DF-WF. A connection from one toll center EF to another toll center EF may, therefore, be effected, in accordance with the geographical position of the two toll centers, over only one direct cross-connection EF--EF, or over two series connections EF--VF-EE or over a combination of two series connections and one cross-connection EF-VF-VF-EF, and so on. The cross-connections EF-EF, the series connections EF--VF, and most of the cross-connections VF-VF consist practically of two-wire lines whereas the other connections consist of four-wire lines and contain echo-Suppressors. The invention will thus be applicable to each connection which is effected over at least one regional center DF.

Fig. 8 shows diagrammatically the Various connections which are possible between two toll centers and which make use of four-wire lines with echo-Suppressors. The full-line transverse arrows represent the echo-Suppressors in their normal position, Whereas the dotted-line arrows designate the echo-Suppressors which are to be reversed or changed-over when two or more four-wire lines are switched through according to the diagram shown.

lThe Connection EF-VF-VF-EF comprises at most one four-wire line so that in this case no change-over is required.

In a connection of the next higher order, EF-VF-DF-VF--EF, the near the regional center DF are to be changedover or reversed.

In a connection EF- VlL-EF--EF-VF-EF the echo-suppressors of the cross-connection- DF-DF are situated, it is evident, in the proper direction so that only the echo-Suppressors of the series connections VF-DF near the terminals at DF must be changed or reversed.

This scheme is to be continued in a manner evident from the further diagrams of Fig. 8.

From this consideration results the principle that changing of echo-Suppressors is necess tated only in the series-connections, and that only at the terminals associated with the toll oiiice of the higher grade, whereas the echo-suppressors in the cross-connections may remain in their normal position.

. This principle, however, must not be followed strictly in all cases. In the case, for example, of a connection EF-VF-DF-WF-DF-VF-EF, as shown in Fig. 8, it may be sufficient to changeover or reverse the echo-Suppressors at the WF- terminals of the two series connections DF-WF, whereas in the series connections of lower grade, VFL-DF, the echo-Suppressors at the terminals of the higher-graduated ofces DF may be fully disabled or disconnected instead of being reversed or changed-over. This is allowable since the other Suppressors are suflcient to secure good control of the entire connection.

The reversal or changing-over of the echo-suppressors in the cases mentioned above may be elected either manually or automatically by fFig.' i7A shows ldiagrammatically the various echo-SuppressorsA switching means which are, on establishing the through-connections at the terminals, actuated in any suitable manner according to the types of the circuits which are to be connected. The switching means used in such cases may be built up in accordance with the principles well known in the art of cord-circuit repeaters.

I claim:

1. In a four-wire telephone transmission circuit including two one-way two-wire transmission paths adapted for transmission in opposite directions, terminals at the two ends of the four-wire circuit each arranged to transform the two oneway paths of the four-wire circuit into a single two-way two-Wire transmission path, two crosssituated echo-Suppressors each of which is located near one of the respective terminals of the fourwire circuit and comprising means operable by voice currents in the incoming path for disabling the outgoing path, and a further pair of echo- Suppressors located between the said terminal echo-Suppressors and connected in such a manner that in each path the branch point of one of these further Suppressors follows the suppression point of the other.

2. In a four-wire telephone transmission circuit including two one-way two-wire transmission paths adapted for transmission in opposite directions, terminals at the two ends of the four-wire circuit each arranged to transform the two one- Way paths of the four-wire circuit into a single two-way two-wire transmission path, two crosssituated echo-Suppressors each of which is located near one of the respective terminals of the fourwire circuit and comprising means operable by voice currents in the incoming path for disabling the outgoing path, and a further pair of echosuppressors located between the said terminal echo-Suppressors substantially at the electrical midpoint of the four-wire circuit and connected in such a manner that in each path the branch point of one of these further Suppressors follows the suppression point of the other.

3. In a four-wire telephone transmission circuit including two one-way two-wire transmission paths adapted for transmission in opposite directions, terminals at the two ends of the four-wire circuit each arranged to transform the two oneway paths of the four-wire circuit into a single two-way two-wire transmission path, two crosssituated echo-Suppressors each of which is located near one of the respective terminals of the fourwire circuit and comprising means operable by voice currents in the incoming path for disabling the outgoing path, and a further pair of echosuppressors with extremely short operating time, located between the said terminal echo-suppressors and connected in such a manner that in each path the branch point of one of these further suppressors follows the suppression point of the other.

4. In a telephone transmission system, a plurality of four-wire transmission lines adapted to be connected together in tandem, two cross-situated echo-Suppressors associated with each of said four-wire lines, each of said echo-Suppressors being located near one cf the two terminals of the respective four-wire line and being responsive to voice currents in the incoming path for disabling the outgoing path, and change-over switching means operating when said four-wire lines are connected together in tandem to reverse some of the echo-Suppressors near the connecting points in such a manner that looking from one terminal in the transmission direction. only suppression points of echo-Suppressors will besituated in the first part of the path and only branch points in the remaining part of the same path.

5. In a telephone toll switching system, a plurality of switch oices of various order, four-wire transmission lines between said switch offices adapted to be connected in tandem, two crosssituated echo-Suppressors associated with each of said four-wire lines, each of said echo-suppressors being located near one of the two terminals of the respective four-wire line and being responsive to voice currents in the incoming path for disabling the outgoing path, and change-over switching means operating when said four-wire lines are connected together in tandem to reverse some of the echo-Suppressors near the connecting points, said switching means being operable only in a four-wire line between two switch ofces of different order, and Within this line only at the terminal associated with the switch oflice of the higher order.

6. In a telephone toll switching system, a plurality of switch offices, four-wire transmission lines between said switch offices adapted to be connected in tandem, two cross-situated echosuppressors associated with each of said four-wire lines, each of said echo-Suppressors being located near one of the two terminals of the respective four-wire line and being responsive to voice currents in the incoming path for disabling the outgoing path, and change-over switching means operating when said four-wire lines are connected together in tandem to reverse some of the echo-Suppressors near the connecting points, said switching means being operable only at the two terminals connected together in the middlemost switch ofi-ice whereas further switching means are provided to disable the other wrongly situated echo-Suppressors in the total four-wire connection.

'7. In a telephone toll switching system, a plurality of switch oiiices, four-wire transmission lines between said switch oices adapted to be connected in tandem, two cross-situated echosuppressors associated with each of said fourwire lines, each of said echo-Suppressors being located near one of the two terminals of the respective four-wire line and being responsive to voice currents in the incoming path for disabling the outgoing path, and change-over switching means operating when said four-wire lines are connected together in tandem to disable all of the echo-Suppressors except the echo-Suppressors near the end terminals, and a cross-situated pair of echo-Suppressors near the electrical midpoint of the total four-wire connection.

8. In a telephone toll switching system, a plu rality of switch offices, four-wire transmission lines between said switch ofces adapted to be connected in tandem, two cross-situated echosuppressors associated with each of said fourwire lines, each of said echo-Suppressors being located near one of the two terminals of the respective four-wire line and being responsive to voice currents in the incoming path for disabling the outgoing path, and change-over switching means operating when said four-wire lines are connected together in tandem to reverse some of the echo-Suppressors near the connecting points controlled by control currents sent out from one of the associated switch ofces, such reversal affecting the connections in such a manner that looking from one terminal in the transmissio-n direction, only suppression points of echo-Suppressors will be situated in the rst part of the path and only branch points in the remaining part of the same path.

9. In a telephone toll switching system, a plurality of switch offices, four-wire transmission lines between said switch oices adapted to be connected in tandem, a pair of oppositely directed echo-Suppressors associated with each terminal of said four-wire lines, one of said echo-suppressors being operative and the other inoperative in the normal state of the line, and switching means operating when said four-wire lines are connected together in ltandem to disable the normally operative echo-suppressor and to render eiective the other.

10. In a telephone transmission system two four-wire transmission lines adapted to be connected together in tandem, two cross-situated echo-Suppressors associated with each of said four-wire lines, each of said echo-Suppressors being located near one of the two terminals of the respective four-wire line and being responsive to voice currents in the incoming path for disabling the outgoing path, and change-over switching means operating when the two fourwire lines are connected together in tandem t reverse the echo-Suppressors near the connecting point of the said two lines in such a manner that in each path the branch point of one of the reversed suppressors follows the suppression point of the other of the reversed suppressors.

11. In a telephone transmission system two four-wire transmission lines adapted to be connected together in tandem, two cross-situated echo-Suppressors associated with each of said four-wire lines, each of said echo-Suppressors being located near one of the two terminals of the respective four-wire line and being responsive to voice currents in the incoming path for disabling the outgoing path, and change-over switching means operating when the two fourwire lines are connected together in tandem to reverse the echo-Suppressors near the connecting point of the said two lines in such a manner that the echo-Suppressors within the same line will operate in the same direction but echo-suppressors in adjoining lines will operate in opposite directions.

12. In a telephone transmission system two four-wire transmission lines adapted to be connected together in tandem, two echo-Suppressors connected in one of said lines to operate in the same direction, each of said echo-Suppressors being located near one of the two terminals of the respective line, and two echo-Suppressors connected in the other of said lines to operate in the direction opposite to the rst-mentioned direction, each of said last mentioned echo-Suppressors being located near one of the two terminals of the other line.

HANS MAYER. 

